Place two markers on a wall, vertically above each other and about 30 cm apart.
Stand with one foot in front of the other, heel to toe.
Look forwards, directly between both markers.
Keep your head still and move your eyes to look up at the top marker.
Focus.
Lift up your head so that you are looking directly at the same marker.
Keep your head still and move your eyes to focus on the bottom image.
Focus.
Move your head down to look directly at this marker.
Continue moving between the two targets in this fashion.
